The invention relates to a floating construction comprising a floating body having a lower part extending below water level and an upper part extending above water level, the floating body being connected to the sea bed by means of at least two substantially parallel connecting elements extending in a substantially straight line between the sea bed and the floating body.
For offshore well operations tension leg platforms (TLP) are used which are moored to the seabed by vertical tethers or tendons, which may be connected to opposite legs of partially submersed parts of the platform. Upon pitch or roll motions of the platforms, large and unevenly distributed tensional forces are exerted on the tethers.
SPAR buoys are also used in the offshore industry for drilling, hydrocarbon storage and/or transfer, a SPAR buoy comprising a slender floating body supporting several deck structures such as a well head deck, a manifold deck, a production well drilling deck and the like. The deep draft SPAR buoys, which may have a height of 150 meters or more, are relatively insensitive to wave induced motions and have a favourable heave and pitch-roll response. Two types of moorings are most prevalent for attaching SPAR buoys to the seabed. These comprise radially spaced catenary anchor lines or taut leg moorings and vertical tether moorings. From the well head, one or more risers extend upward to the SPAR buoy for transferring hydrocarbons from the subsea well. The risers may be flexible or may comprise a rigid steel casing.
According to one known construction, the risers extend along the outside of the floating SPAR buoy and are fixed to the riser attachment deck. A tensioned tether is attached to the lower end of the SPAR buoy such that the natural heave period generally is less than 5 seconds. Upon drift of the SPAR buoy, the tether will be displaced from its vertical position. Due to mean and dynamic wave motions, a relative angular motion (pitch-roll) of the SPAR buoy and the tether occurs which will cause a slackening of the risers on one side of the tether and an overtensioning of the risers on the opposite side of the tether. This unequal load distribution may lead to fatigue weakening which may result in failure of the risers.
From U.S. Pat. No. 4,702,321 a free floating SPAR construction is known wherein the riser movement is decoupled from the SPAR buoy movements. In the SPAR buoy that is described in the above mentioned document, a number of risers extend upwards through the central well of the floating SPAR body to a dry production deck. Each riser is at its upper end buoyantly supported by a buoyancy tank situated around the respective riser. As the risers are free at their upper ends, they can axially slide up and down in the well. The SPAR buoy is moored to the seabed by taut lateral moorings, such that the natural heave period of the known construction is larger than 25 seconds. When the known SPAR buoy is tilted, overtensioning of the risers is prevented by the axial sliding motion of the risers within the well. However, the riser motion inside the well causes significant wear. Furthermore, the known construction has a relatively large diameter in order to accommodate the riser buoyancy tanks and is therefore relatively sensitive to current and wave induced motions. Also, in case of rupture of one of the risers, the hydrocarbons will spill into the confined well. In view of the absence of natural ventilation, this may result in a danger of explosions.
It is therefore an object of the present invention to provide a mooring construction that avoids overtensioning of the elements extending between the sea bed and the floating body, which is of a relatively simple construction and which has a relatively small volume.
It is a further object of the present invention to provide a mooring construction which has a stable deck orientation.
It is again another object of the present invention to provide a mooring construction which can be easily installed.
It is a further object of the present invention to provide a mooring construction wherein the inclination of the floating body from the vertical and the relative angle between the vertical center line of the floating body and the connecting elements may be limited.
It is again an object of the present invention to provide a mooring construction that limits the forces on the connecting elements during extreme weather conditions.
For this purpose, a floating construction according to the present invention is characterised in that the floating body comprises a mounting frame to which the upper parts of the connecting elements are movably attached, and a displacement member attached to the mounting frame and to the end parts of two connecting elements that are placed on respective sides of a vertical center line of the mounting frame for causing oppositely directed and substantially equal displacements of the connecting elements relative to the mounting frame upon tilting and/or a sideways excursion of the floating body to maintain a substantially similar tension in the connecting elements.
During a sideways excursion and/or during roll or pitch of the floating construction according to the present invention, the connecting elements will be maintained in a parallel relationship by the displacement member. Via the displacement member, one of the connecting elements is raised while the other is lowered by the same amount, such that the upper parts of two connecting members that are located on opposite sides of the longitudinal center line are maintained in a substantially horizontal plane. Hereby an overtensioning of the connecting elements is effectively prevented.
The present invention may be used for floating constructions such as mooring buoys, tension leg platforms, tankers and the like. The invention is particularly suitable for use in conjunction with SPAR buoys, which generally have a length dimension along the vertical center line which is at least five times larger than the width dimension. In this case the connecting elements, such as risers and/or tethers are placed on separate sides of the vertical center line of the SPAR buoy, that coincides with the vertical center line of the mounting frame. By the present invention overtensioning of the connecting elements, which may comprise risers and/or tethers, is prevented upon inclination of the center line of the SPAR buoy from the vertical.
In other embodiments, the vertical center line of the mounting frame does not coincide with the vertical center line of the vessel, for instance in case the mounting frame is placed on one side of a vessel.
In one embodiment of the floating construction according to the present invention, the displacement member comprises a pivotable arm. As used herein the term xe2x80x9carmxe2x80x9d may also comprise a two-dimensional structure such as a deck construction. Each connecting element is pivotably connected to a respective end of the arm. The pivot arm allows limited vertical movement of the connecting elements while transferring excess tension in one connecting element to the other element having a smaller tension. In this manner a simple mechanism is provided for keeping the ends of the connecting elements in a substantially horizontal plane upon drift and/or pitch and roll of the floating body.
The connecting elements according to the present invention may comprise risers, tethers or both. When sufficiently strong, steel hard piping is used for the risers, it is envisaged that the floating body according to the invention may be anchored to the seabed by the use of the risers only. However, the floating body according to the present invention is preferably used in conjunction with one or more tethers, which may be connected to the floating body via a fixed or via a pivoting connection, which may include the mounting frame.
A preferred embodiment of a floating construction according to the present invention has a displacement member with at least two arms that are pivotably connected to one end of a respective hydraulic or pneumatic cylinder which is connected to the mounting frame. Each arm carries at its free end a connecting element. The hydraulic or pneumatic cylinders are mutually connected by a fluid duct. A constant volume of fluid is displaced between the cylinders upon movement of the floating body. Hereby the cylinders are actuated in opposite directions such that the tension in the connecting elements is substantially equalised. The use of pressure fluid cylinders is particularly useful upon installation of the risers, wherein additional cylinders can be added to the mounting frame as new risers are being put in place.
In another embodiment of a floating construction according to the present invention, the displacement member comprises two cable guide members, each connecting element being with its upper end attached to a respective end of a cable that extends from the first connecting element, via the cable guide members, to the second connecting element. The cable and cable guides form a relatively simple and light weight construction to maintain the connecting elements in a uniform tensile situation. The cable may comprise an elastic section, for instance a spring element to compensate for small secondary misalignments between the connecting elements that may be caused by bending of the riser casing near the lower part of the floating body such as at the SPAR/tether pivot, or by height variations in the seabed.
The floating structure may comprise a pivotable deck which is pivotably coupled to the connecting elements. The deck itself may act as the displacement member for the connecting elements, or may be pivotably connected to the displacement member. As the displacement member and the geometry of the parallel connecting elements cause the upper parts of the connecting elements to be located in a substantially horizontal plane, irrespective of the inclination of the floating body, the displacement member can effectively be used to keep a deck structure, such as the riser attachment deck, in a horizontal position.
In another embodiment of a floating construction according to the present invention, the mounting frame is situated at or near the upper part of the buoy, a guide frame being connected at or near the lower part. The connecting elements are guided through respective passages in the guide frame. The guide frame maintains the connecting elements in their proper position with respect to the floating body, and prevents the floating body from contacting the risers or tethers upon tilting.
The guide frame may be fixed to the floating body in a stationary manner, in which case a sliding movement of the connecting elements through the guide frame can occur. Alternatively, the guide frame is pivotably connected to the floating body. In this way, the guide frame remains properly aligned with respect to the connecting elements, and bending is reduced. Furthermore, by pivoting of the guide frame, the sliding movements of the connecting elements are reduced and a reliable operation during extreme weather conditions is achieved.
The guide frame may for each connecting element comprise a sleeve which is pivotably attached to a radial arm connected to the lower part of the floating body. The sleeves prevent excessive bending or buckling of the connecting elements. To prevent damage to the connecting elements, the sleeves can near their edges be provided with a relatively soft, preferably replaceable lining material. For the same reason, the connecting elements can in the region of the sleeves be provided with a contact member for contacting the internal wall of the sleeves.
To further reduce the angle of curvature of the connecting elements, the radial arms of the guide frame are connected to a central pivot at or near the longitudinal axis of the floating body, at or near the lower part thereof. A number of sleeves may be placed on a circular guide frame at spaced angular positions to accommodate a circular configuration of the connecting elements.
In a preferred embodiment, at least one tether is attached to the floating body. By the use of tethers a small natural period of the floating construction is achieved which is outside the region of significant wave excitation. Preferably the tethers are also passed through the guide frame to cause controlled bending. The tethers may be attached to the lower part of the floating body, to a pivoting part of the guide frame, to the upper part of the floating body or to the mounting frame. In a SPAR embodiment according to the present invention, the tethers and the risers are preferably alternatingly placed in a circular pattern so that they can be installed outside the SPAR body. This tether configuration also gives a better control of the horizontal movements of the SPAR.
The floating construction according to the present invention may be connected to the seabed via a template. Preferably the template has a compression space allowing an axial depression of the at least one tether. If during extreme weather conditions the tethers are depressed, buckling is prevented as the tethers can move down into the compression space.
In addition to the tethers, the floating construction may comprise at least two pairs of axially spaced mooring lines. In one embodiment, each of the spaced mooring lines has a first section extending from the seabed to a first guide element on the floating body, a second section extending axially from the first guide element to second guide element and a third section extending from the second guide element back in the direction of the seabed or towards the first section. By the use of the radial mooring, the SPAR and tether angles from the vertical and the relative angle between the SPAR and the tether can be minimised. Reduction of these angles reduces bending fatigue in the risers and tethers. Furthermore, the adjustment of the mooring lines, by one or more winches on the SPAR buoy, can bring the SPAR and tethers to a vertical orientation, which is favourable when drilling wells on the SPAR.